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    <p>Elaboration on elclanrs' answer.</p> <p><code>foo.bar</code> is a method; an instruction of how to do something. In object-oriented programming languages, the only object that can use that instruction is a <code>foo</code> or an object related to <code>foo</code>; but in JavaScript, anybody can try to run it if the code asks them to.</p> <p>In your second anonymous function, you are getting a method to run, running it, and returning the type of the result. However, it's not a <code>foo</code> that is running this function; it's anonymous, so the <code>window</code> is the object that's running it. The <code>window</code> runs <code>foo</code>, which tries to return <code>this.baz</code>; however, the <code>window</code> doesn't HAVE a <code>baz</code>, so that's why you're getting undefined.</p> <p>To test it further, try setting <code>window.baz</code> and see if you get the right result, and also try elclanrs' suggestion of using the <code>call()</code> method to ensure it gets called from a <code>foo</code>'s scope.</p> <p><strong>EDIT</strong></p> <p>You're right, the type of <code>arguments[0]</code> and <code>foo.bar</code> IS the same; they're both "function". I think the confusion is over JavaScript's treatment of functions as first-class objects.</p> <p>If you're familiar with object-oriented languages like Java or C++, then you would expect that when you call <code>bar</code>, it's always being called by a <code>foo</code> that has a <code>baz</code> property; however, this isn't the case in JavaScript. Any function can be called by ANY object, which means the result may or not make sense.</p> <p>The difference between the two, as elclanrs says, is all about the <code>.</code>. Let's say I made my own object <code>phew = {baz: 44}</code>. I could, validly, "steal" a method from <code>foo</code> like this: <code>phew.myMethod = foo.bar</code>. Now, <code>phew</code> knows the instructions contained in <code>bar</code>, and if I call <code>phew.myMethod()</code>, I would get 44 as my result because <code>phew</code> is calling the method, and <code>phew</code>'s <code>baz</code> is 44; it doesn't matter <em>where</em> the method is defined! All that matters is <em>what the method says to do</em>, and <code>bar</code> says to return the <code>baz</code> of whoever called it.</p> <p>So now, going back to your code, you're calling <code>arguments[0]()</code>. It does seem like it should be the same, but because functions are first-class objects, when you passed it as a parameter, all you really passed was a function called <code>bar</code>. When you called <code>arguments[0]()</code>, it was like calling <code>bar()</code>, which is different from calling <code>foo.bar()</code> just like it's different from calling <code>phew.myMethod()</code>, <strong>even though they all are exactly the same function</strong>. </p> <p>For more info, try <a href="https://stackoverflow.com/questions/705173/what-is-meant-by-first-class-object">this SO post</a></p>
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    1. POdid a little javascript experiment, needs your help to understand the unexpected results
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    1. COActually you didn't answered my question. What I don't understand is how `arguments[0]` id different from `foo.bar` ? Both returns the same function `function () { return this.baz; }` then why `arguments[0]()` use global object but the `foo.bar()` not?
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    2. CO`arguments[0]()` is different from `foo.bar()` because it first valuate `arguments[0]` as `function () { return this.baz; }` which is not a method of object `foo`. While `foo.bar()` runs as a method of `foo`.
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    3. COActually if you put console.log(foo.bar === arguments[0]) inside the anonymous function it shows true. where as if you write two same function they will not be equal.
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